Negative-acting, liquid-type photoresist compositions based upon cyclized rubber and containing diazide sensitizers are well known and are commonly employed in lithographic applications to form printing plate images. Their commercial acceptance in microelectronic applications, such as for creating micron size images on silicon wafers has, however, been limited by a number of technical problems. These problems include difficulties in obtaining high quality, pinhole-free, coatings of good uniformity and adhesion on substrate surfaces without the need for special surface preparation or other processing steps; inadequate thermal stability and resolution capability for the resulting images to be useful in many microelectronic applications, and especially in their need for organic solvents for developing the negative image.
For example, con entional negative acting photoresists employ rubbery thermoplastic polymers, such as polyisoprene and cyclized rubber, which require the use of organic solvents for development. When such negative resists are developed it has been found that the exposed thermoplastic material will swell in the organic solvent developer. The resolution of the images are thereby compromised and in some instances the images will become distorted and therefore unusable. Organic solvent developers are also undesirable for environmental, health and flammability reasons.
"DCOPA" is a copolymer containing glycidyl methacrylate and 2,3-dichloropropylmethacrylate. It has been reported to be useful as an x-ray sensitive photoresist material in "Recent Printing and Registration Results with X-Ray Lithography, B. Fay, L. Tai and D. Alexander, SPIE Vol. 537 (1985) pg. 57-68. DCOPA is a soft rubbery material which softens at low temperatures, on the order of less than 100.degree. C., and is insufficiently resistant to plasma etching conditions used in subsequent wafer processing. This plasma etch resistance problem is inherent in aliphatic polymeric materials.
Novolak containing photoresist materials are substantially aromatic materials which exhibit good plasma etch resistance; however, they are not thermally stable nor are they sensitive to x-rays and other short wavelength radiation.
Workers in the microlithographic art are therefore seeking improved photoresists which are sensitive to short wavelength actinic radiation, are capable of high resolution, and are thermally resistant and resistant to plasma etching.
In my copending application Ser. No. 616,518, I described a dual acting, positive or negative, photosensitive composition containing an acid hardening resin system and a photoacid generator useful in the near ultraviolet radiation spectrum for preparing aqueously developable, thermally stable images. These dual acting photosensitive compositions result in images which are substantially more thermally stable than images formed by using conventional photoresist compositions such as those formed from novolak resins. The photoacid generators used in the dual acting photosensitive compositions of my previous invention were limited to those which were sensitive to near ultraviolet radiation ("near UV") having wavelengths on the order of form about 300 to about 500 nanometers and more particularly about 365 nanometers. Those near UV photoacid generators, such as diazonaphthoquinones, yield weak carboxylic acids, such as indenecarboxylic acid, when the photosensitive composition is exposed to near UV radiation. Those photoacid generators are typically present in the photosensitive composition at a concentration of from about 10 to 30 weight percent based on the total solids content of the photosensitive composition. The carboxylic acid photoacid generators used in combination with acid hardening resin systems of my prior invention were not found, however, to be sensitive to shorter wavelengths such as, deep ultraviolet radiation on the order of from about 210 to about 300 nanometers and more particularly 254 nanometers ("deep UV").
Compounds which generate strong inorganic acids, such as hydrochloric acid upon exposure to near UV radiation, as disclosed in U.S. Pat. Nos. 3,692,560; 3,697,274; 3,890,152; 4,404,272, were found to be unsuitable for use with acid hardening resins for aqueously developing both positive and negative, thermally stable images.
It was an object of the present invention, therefore, to provide photosensitive compounds, more properly referred to as photoacid generators, which are sensitive to short wavelength radiation such as deep UV or x-ray radiation and which can be used in combination with acid hardening resin systems for preparing aqueously developable, thermally stable and highly resolved negative images suitable for use in microelectronics applications.